1. Field of the Invention
The present invention relates to a power transmission system of a boat propulsion unit including a transmission unit that receives an output from an engine, changes its speed, and transmits the output to a propeller. More specifically, the present invention relates to a water cooling apparatus for cooling the transmission unit with cooling water.
2. Description of the Related Art
The above type of conventional boat propulsion unit is disclosed in WO 2007/007707, which discloses a boat propulsion unit including a case which is disposed right behind a hull and is supported at the rear of the hull, a propeller supported in a lower portion of the case, and a transmission unit housed in the case to receive an engine output, change its speed, and transmit the engine output to the propeller.
In an attempt to propel a boat, if an engine output is input to the transmission unit of the power transmission system, the speed of this input is changed to a desired state in the transmission unit based on a speed change operation by a crewmember and is transmitted to the propeller. Then, the boat travels at a speed corresponding to the speed of the propeller.
The transmission unit tends to be heated by frictional heat from a rotating body thereof during a speed change operation with the transmission unit. Such heating of the transmission unit affects the lifetime of the transmission unit.
A cooling water passage is formed in a transmission case forming an outer shell of the transmission unit to flow cooling water therein in order to prevent the transmission unit from being heated. In this case, a large amount of water is required to flow through the cooling water passage to sufficiently cool the transmission unit. Therefore, the water cooling passage is formed in the transmission case with a relatively large cross-sectional area. However, the larger cross-sectional area of the water cooling passage may outwardly expand an outer surface of the transmission case. An outer surface of the case that houses the transmission unit may also expand outward accordingly.
As described above, the case of the boat propulsion unit is disposed right behind the hull and is supported at the rear of the hull. Therefore, if the outer surface of the case expands outward as described above, the center of gravity of the boat propulsion unit tends to recede rearward from the rear of the hull in order to avoid interference of the case with the rear of the hull. As a result, the support moment by which the rear of the hull supports the boat propulsion unit increases, thereby causing a problem that strain on the rear portion of the hull increases.